The present invention relates to multilayer optical films having two or more different sets of layers, each set being formed from a different polyester, and to improved polyesters for use in these films.
Polymeric films arc used in a wide variety of applications. One particular use of polymeric films is in mirrors and polarizers that reflect light of a given polarization and wavelength range. Such reflective films are used, for example, in conjunction with backlights in liquid crystal displays to enhance brightness and reduce glare of the display. A polarizing film may be placed between the user and the backlight to direct the light towards the user and to polarize the light; thereby reducing the glare. In addition, a mirror film may be placed behind the backlight to reflect light towards the user; thereby enhancing brightness. Another use of polarizing films is in articles, such as sunglasses, to reduce light intensity and glare.
One type of polymer that is useful in creating polarizer or mirror films is a polyester. One example of a polyester-based polarizer includes a stack of polyester layers of differing composition. One configuration of this stack of layers includes a first set of birefringent layers and a second set of layers with an isotropic index of refraction. The second set of layers alternates with the birefringent layers to form a series of interfaces for reflecting light. The polarizer may also include one or more non-optical layers which, for example, cover at least one surface of the stack of layers to prevent damage to the stack during or after processing. There are other configurations that may also be used in polarizer/mirror films including stacks of layers with two or more different sets of birefringent and/or isotropic layers.
The properties of a given polyester are typically determined by the monomer materials utilized in the preparation of the polyester. A polyester is often prepared by reactions of one or more different carboxylate monomers (e.g., compounds with two or more carboxylic acid or ester functional groups) with one or more different glycol monomers (e.g., compounds with two or more hydroxy functional groups). Each set of polyester layers in the stack typically has a different combination of monomers to generate the desired properties for each type of layer. There is a need for the development of polyester films for use in polarizers and mirrors which have improved properties including physical properties, optical properties, and lower manufacturing cost.
Generally, the present invention relates to a multilayered polymer film. One embodiment is a multilayered polymer film which includes a plurality of first layers and a plurality of second layers. The first layers are made with a first copolyester which is semicrystalline and birefringent. The first copolyester includes carboxylate subunits and glycol subunits in which 70 to 100 mol % of the carboxylate subunits are first carboxylate subunits, 0 to 30 mol % of the carboxylate subunits are first comonomer carboxylate subunits, 70 to 100 mol % of the glycol subunits are first glycol subunits, and 0 to 30 mol % of the glycol subunits are first comonomer glycol subunits, and at least 2.5 mol % of the combined carboxylate and glycol subunits of the first copolyester are first comonomer carboxylate subunits, first comonomer glycol subunits, or a combination thereof. The second layers are made with a second polymer which has an in-plane birefringence of about 0.04 or less, at 632.8 nm, after the multilayered polymer film has been formed.
Another embodiment is a multilayered polymer film having a plurality of first layers and a plurality of second layers. The first layers are made with a first copolyester which is semicrystalline and birefringent. The first copolyester includes carboxylate subunits and glycol subunits in which 70 to 100 mol % of the carboxylate subunits are first carboxylate subunits, 0 to 30 mol % of the carboxylate subunits are first comonomer carboxylate subunits, 70 to 100 mol % of the glycol subunits are first glycol subunits, 0 to 30 mol % of the glycol subunits are first comonomer glycol subunits, and at least 0.5 mol % of the combined carboxylate and glycol subunits of the first copolyester are first comonomer carboxylate subunits, first comonomer glycol subunits, or a combination thereof. The first copolyester has in-plane indices of refraction which are 1.83 or less and which differ by 0.2 or greater when measured with 632.8 nm light. The second layers are made with a second polymer which has an in-plane birefringence of about 0.04 or less at 632.8 nm after the multilayered polymer film has been formed.
A further embodiment is a multilayered polymer film which has a plurality of first layers and a plurality of second layers. The first layers are made with a first copolyester which is semicrystalline and birefringent. The first copolyester has carboxylate subunits and glycol subunits in which 70 to 100 mol % of the carboxylate subunits are first carboxylate subunits, 0 to 30 mol % of the carboxylate subunits are first comonomer carboxylate subunits, 70 to 100 mol % of the glycol subunits are first glycol subunits, 0 to 30 mol % of the glycol subunits are first comonomer glycol subunits, and at least 0.5 mol % of the combined carboxylate and glycol subunits of the first copolyester are first comonomer carboxylate subunits, first comonomer glycol subunits, or a combination thereof. The second layers are made with a second polymer having an in-plane birefringence of about 0.04 or less at 632.8 nm after the multilayered polymer film has been formed. The multilayered polymer film is formed by drawing the first and second layers in at least one draw direction to a particular draw ratio. After drawing the first and second layers, the first layers of the multilayered polymer film have an index of refraction in, the draw direction which, at 632.8 nm, is at least 0.02 units less than an index of refraction in the draw direction of a similarly constructed polyethylene naphthalate layer which has a same in-plane birefringence and draw ratio.
Yet another embodiment is a multilayered polymer film which includes a plurality of first layers and a plurality of second layers. The first layers are made with a first copolyester which is semicrystalline and birefringent. The first copolyester has carboxylate subunits and glycol subunits in which 70 to 100 mol % of the carboxylate subunits are first carboxylate subunits, 70 to 99 mol % of the glycol subunits are first glycol subunits, and 1 to 30 mol % of the glycol subunits are first comonomer glycol subunits. The second layers are made with a second polymer which has an in-plane birefringence of about 0.04 or less, at 632.8 nm, after the multilayered polymer film has been formed.
Another embodiment is a multilayered polymeric film which has a plurality of first layers and a plurality of second layers. The first layers are made with a first copolyester which is semicrystalline and birefringent. The second layers are made with a second copolyester which has an in-plane birefringence of about 0.04 or less, at 632.8 nm, after the multilayered polymer film has been formed. The second copolyester includes carboxylate subunits and glycol subunits in which 0.01 to 2.5 mol % of the combined carboxylate and glycol subunits are derived from compounds with three or more carboxylate or ester functional groups, compounds with three or more hydroxy functional groups, or a combination thereof.
A further embodiment is a multilayer polymer film which includes a plurality of first layers and a plurality of second layers. The first layers are made with a first copolyester which is semicrystalline and birefringent. The second layers are made with a second copolyester of polyethylene naphthalate. The second copolyester includes glycol subunits and carboxylate subunits in which the glycol subunits are 70 to 100 mol % ethylene or butylene subunits and about 0 to 30 mol % comonomer glycol subunits derived from one or more of 1,6-hexanediol, trimethylol propane, or neopentyl glycol, and the carboxylate subunits are 20 to 100 mol % naphthalate subunits, 0 to 80 mol % terephthalate or isophthalate subunits or mixtures thereof, and 0 to 30 mol % of comonomer carboxylate subunits derived from phthalic acid, t-butyl-isophthalic acid, lower alkyl esters of these acids, or a combination thereof; and at least 0.5 mol % of the combined carboxylate and glycol subunits of the second copolyester are comonomer carboxylate subunits, comonomer glycol subunits, or a combination thereof.
Another embodiment is a polymer which is a copolyester having an intrinsic viscosity of about 0.4 dL/g or greater as measured in a 60/40 wt. % mixture of phenol/o-dichlorobenzene. The polymer includes glycol subunits and carboxylate subunits in which the glycol subunits are 70 to 99 mol % ethylene or butylene subunits and 1 to 30 mol % comonomer glycol subunits derived from 1,6-hexanediol, and the carboxylate subunits are 5 to 99 mol % naphthalate subunits, 1 to 95 mol % terephthalate or isophthalate subunits or a combination thereof, and 0 to 30 mol % of comonomer carboxylate subunits derived from phthalic acid, t-butyl-isophthalic acid, lower alkyl esters of these acids, or a combination thereof; and at least 0.01 to 2.5 mol % of the combined carboxylate and glycol subunits of the copolyester are derived from compounds having three or more carboxylate, ester, or hydroxy functional groups.
One other embodiment is a multilayer polymer film which includes a plurality of birefringent first layers and a plurality of second layers. The first layers are made with a first copolyester having naphthalate subunits. The second layers are made with a second copolyester which has an intrinsic viscosity of 0.4 to 0.5 dL/g and contains 0.01 to 5 mol % comonomer subunits which are derived from compounds having three or more carboxylate, ester, or hydroxy functional groups. The multilayer polymer film also includes one or more non-optical layers that have an intrinsic viscosity of 0.5 dL/g or greater.
The above summary of the present invention is not intended to describe each illustrated embodiment or every implementation of the present invention. The figures and the detailed description which follow more particularly exemplify these embodiments.
The invention may be more completely understood in consideration of the following detailed description of various embodiments of the invention in connection with the accompanying drawings, in which:
FIG. 1 is a cross-sectional view of one embodiment of a multilayered polymer film according to the present invention;
FIG. 2 is a cross-sectional view of another embodiment of a multilayered polymer film according to the present invention;
FIGS. 3A and 3B are graphs illustrating the decrease in glass transition temperature (FIG. 3A) and freezing temperature (FIG. 3B) with the addition of terephthalate (using dimethyl terephthalate (DMT)) and isophthalate (using dimethyl isophthalate (DMI)) subunits to polyethylene naphthalate (PEN) which is derived from dimethyl naphthalene dicarboxylate;
FIG. 4 is a graph of the average in-plane birefringence of coPEN modified with terephthalate and isophthalate subunits and oriented at relatively low temperatures;
FIG. 5 is a graph of the thermal stability of coPEN containing terephthalate and isophthalate subunits;
FIG. 6 is a graph illustrating the reduction in in-plane birefringence, at 632.8 nm, of a coPEN by the addition of comonomer subunits; and
FIG. 7 is a graph illustrating the dependence of in-plane birefringence, at 632.8 nm, on molecular weight.